This invention concerns a novel method of thermoplastically reducing the diameter of metal tubes. The inventive method is of particular utility in conjunction with the manufacture of dual wall tubes or pipes, such as wear-resistant piping for the transportation of fluids therethrough, by reducing the diameter of the outer pipe member to shrink on the inner pipe member.
Pipelines constitute a major means of transportation of all sorts of fluids, as well as fluidized solids, in a wide variety of industries. Among the pipelined materials are the slurries (mixtures with water) of such solids as coal, minerals and cement. Finer solid particles such as dust and quartz sand are conveyed by air pressure. A problem encountered with the transportation of such fluidized solids is the abrasion and rapid wear of the inside of the pipe.
A known solution to this problem has been the use of cheap steel pipe of the class used as gas pipe. When worn out, such cheap pipe has been replaced with new one, or patches have been welded to the worn parts. However, in applications where resistance to wear is of utmost importance, pipe of more expensive material such as cast stainless steel of high chromium content has also been in use.
Generally, the wear resistance of steel materials depends in a large measure on their hardness. Highly wear resistant materials are invariably very hard. Some cast stainless steel used as a wear-resistant pipe material has a Shore hardness of as much as 81 or more. The higher the hardness, however, the lower is the ductility of the steel materials. Pipe cast from the noted stainless steel of high chromium content is susceptible to breakage under impact loading.
Additional weaknesses of hard, wear resistant steel materials are their poor weldability and machinability. Pipe sections made of such materials do not allow the welding of flanges thereto. Even if the pipe sections have flanges formed in one piece therewith, difficulties are encountered in boring, finishing or like subsequent machining of the flanges. Pipe repair by welding is also not easy. Still further the manufacturing costs are very high.
Recently, therefore, there has been suggested and used steel pipe having wearproof linings. Such lined pipe has been made by centrifugal casting or by buildup welding, with the linings metallurgically joined to the pipe proper. The pipe with the wearproof linings is far superior in wear resistance to, and thus has a much longer useful life than, the ordinary steel pipe. An additional advantage is that the pipe proper need not be wear resistant and so can be fabricated from sufficiently ductile, weldable materials. The use of such materials makes possible the provision of pipe that, unlike the one made solely from wear resistant materials, is sufficiently strong against impact loading and which allows the welding on of flanges.
Offsetting these advantages of steel pipe having wearproof linings is its susceptibility to cracking because the linings have residual tensile stresses regardless of the method of manufacture. The cracks that have developed in the linings are easy to propagate to the pipe proper, the latter being metallurgically joined to the linings as aforesaid.
What is currently believed to be the best substitute for the lined steel pipe is dual wall pipe consisting of two pipe members of different diameters nested one within the other. The outer pipe member is of a material having practically sufficient ductility whereas the inner pipe member is of a wearproof material. The outer and inner pipe members should not be metallurgically joined to each other but should be self tightened, that is, in contact under pressure, with the inner pipe member under compressive stress. Such dual wall pipe possesses the same advantages as, and is free from the disadvantages of, the above described lined pipe.
Three methods have so far been suggested for the fabrication of the self tightened dual wall pipe. The first is to thermally shrink the outer pipe member onto the inner member; the second is to hydraulically expand the inner member against the outer member; and the third is to thermally expand the outer member and hydraulically expand the inner member against the outer member. All these known methods have the following drawbacks.
The first method requires the inside diameter of the outer pipe member and the outside diameter of the inner pipe member to be machined to very stringent tolerances. However, if the inner pipe member is of a wear resisting material and so is very hard, its machining to the required dimensional tolerances is difficult. The manufacture of elongate dual wall pipe of constant contact pressure is also very difficult by this method.
The second and third methods are analogous in that the inner pipe member is plastically expanded in diameter against the outer member. In the case of dual wall pipe in which the inner pipe member has very high yield strength and is thick, extremely high pressures are required for the diametral expansion of the inner pipe member so that both of these methods are unpractical. In the second method (the hydraulic expansion of the inner member against the outer member) in particular, the inner member has a higher yield strength than the outer member, so that the plastic expansion of the inner member is inevitably followed by some difference in elastic contraction between the two members, resulting in the creation of a gap between the inner and outer members.
A further weakness of the second and third methods is that they are adoptable only in cases where the inner pipe member is metal made. Ceramics have recently been proposed as materials for the inner member of dual wall pipe by virtue of their strength, lightness, and resistance to wear and corrosion. Ceramics are of course nonexpansible, either hydraulically or thermally.
Thus, despite the strong demands expressed by the various industries for dual wall pipe with a wear resisting capability, no truly satisfactory method of manufacture has so far been available.